How does DKIM signing work step-by-step?

This is what happens when you send a signed message:

Your mail system selects a DKIM selector.

It loads the private key.

It hashes the message body.

It hashes specific headers.

It combines those elements to create a cryptographic signature.

It adds the DKIM Signature header to the email.

The recipient later verifies that signature using the public key in DNS.

All of this happens behind the scenes during the voyage.